This invention relates to a metering valve, and more particularly to a metering valve which is suitably used for meter-out control of a construction equipment, an industrial equipment, an agricultural implement or the like.
A conventional metering valve is generally constructed in such a manner as shown in FIG. 1. More particularly, the conventional metering valve includes a valve case 1 which is formed with an actuator port 2, a tank passage 3, and a piston chamber 4 coaxial with the actuator port 2 and closed with a plug 5. In the piston chamber 4 is arranged a piston body 6 in a manner to be slidable therein, and a first pilot chamber 7 is defined between the piston body 6 and the plug 5.
Also, the piston body 6 is formed on a side thereof opposite to the other side thereof facing the first pilot chamber 7 with a poppet section 6a, which is adapted to be pressedly abutted against a seat 9 of the valve case 1 provided on a side of the actuator port 2 by action of a spring 8 arranged in the first pilot chamber 7. The piston body 6 is formed on a periphery thereof with a step 6b and correspondingly the piston chamber 4 is formed on an inner periphery thereof with a step 4a in a manner to be opposite to the step 6b, which cooperates with the step 6b of the piston body 6 to define a second pilot chamber 10 therebetween.
The actuator port 2 is kept fully closed by applying a pilot pressure to the first pilot chamber 7 from pressure source (not shown) and communicating the second pilot chamber 10 to a tank (not shown). More particularly, application of the pilot pressure to the first pilot chamber 7 causes the piston body 6 to be forcedly moved in a left direction in FIG. 1, so that the poppet section 6 may be closely abutted against the seat 9. This results in communication between the actuator port 2 and the tank passage 3 being interrupted to keep the metering valve in a loaded condition.
Full opening of the actuator port 2 is accomplished by applying pilot pressure to the second pilot chamber 10 and communicating the first pilot chamber 7 to the tank. Application of the pilot pressure to the second pilot chamber 10 causes the piston body 6 to be moved against the spring 8 in cooperation with pressure acting on the poppet section 6a on the side of the actuator port 2. Such movement of the piston body 6 causes the poppet section 6a to be separated from the seat 9 to open the actuator port 2, so that the actuator port 2 may be communicated to the tank passage 3. A degree of opening of the actuator port 2 may be controlled by adjusting the pilot pressure in the second pilot chamber 10.
Unfortunately, the conventional metering valve constructed as described above has a disadvantage that a variation of pressure loaded on the side of the actuator port 2 causes meter-out control characteristics of the valve to be varied.
More particularly, for example, in FIG. 1, when pressure loaded on the side of the actuator port 2, pilot pressure applied to the second pilot chamber 10, a pressure receiving area of the poppet section 6a and a pressure receiving area of the second pilot chamber 10 are indicated at P1, P2, A1 and A2, respectively, force by which the piston 6 is moved in a direction indicated at an arrow 11 in the case that the actuator port 2 is to be opened is P1.multidot.A1 +P2.multidot.A2.
Thus, in the conventional metering valve, a variation of the pressure A1 causes a variation of force in the direction of the arrow 11, which then causes a degree of opening of the actuator port 2 to be varied, so that the valve fails to exhibit stable control characteristics.